Preheat oven to 350°F. Line 2 baking sheets with parchment paper or silicone baking mats. Set them aside for now.
Using a hand or stand mixer, cream together the softened butter, granulated sugar, and dark brown sugar for 1-2 minutes, until light and fluffy. Add in the eggs and vanilla extract, and mix until combined.
Add in the 2 ½ cups of flour, baking soda, and salt. Mix until all the floury bits are gone and your dough has formed. Now it's time to divide the dough in two.
You're going to need to divide your cookie dough equally in half, so I do recommend using a kitchen scale to weigh out your dough if you have one. If you don't have one, do your best to divide the dough equally. Place each half into its own bowl. To one half of the dough, add in the remaining 2.5 tablespoons of flour. To the other half, add the 4 tablespoons of cocoa powder. Mix each dough until the flour and cocoa powder are completely mixed in. Fold in 3 oz of chopped semisweet chocolate to each dough.
To form the cookies, you will need a small cookie scoop for each of the two doughs. Each cookie will have two small scoops of dough---- you will use one small scoop of regular chocolate chunk, and one small scoop of the double chocolate chunk. Take a scoop of each dough and press them together. Using your hands, pull and fold the dough in on itself once or twice, then roll it into a ball. This is what will get you the marbled look.*
Place the marbled dough balls onto your prepared baking sheets a few inches apart. Once they're all rolled out, bake the cookies for 11-12 minutes. Once out of the oven, you can press a few more chocolate chunks into the tops of each cookie, if desired. Allow the cookies to cool on the pan for a few minutes, then carefully transfer them to a cooling rack to finish cooling.
Notes
*If you just roll each of the balls of dough together, you won't get a marling effect, you will get more of a yin and yang look to your cookies.